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All posts for the month October, 2015

I’m gearing up for the K2 Science Conference next week and preparing my presentation. So this week at journal club, I thought it would be fun for everyone to give short presentations on their research projects.

Not surprisingly, when so many planets are packed into such a tight space, bad things can happen, and Boley and colleagues showed that such planets often collide with one another, sticking together to form even larger planets. In some cases, the newly formed planet can be large enough that it can accrete gas from its maternal protoplanetary disk and form a gas giant planet.

The standard model for planet formation suggests gas giants shouldn’t form close to their host stars, but Boley and colleagues argue that their collisional scenario could explain the presence of so many hot Jupiters and Neptunes found around Sun-like stars in the last few decades. Their work could help resolve the puzzle of hot Jupiters, an exoplanet mystery older than some of my students.

Journal club attendees included Jennifer Briggs, Emily Jensen, Karan Davis, Tyler Gordon, and Jacob Sabin. (Physics majors Jake and Steve also attended, but I don’t know their last names.)

Even harder to understand, sub-Neptunes display a very broad range of densities, with some having densities greater than Earth’s and others with the density of wind-packed snow. This diversity indicates some planets have large rocky/icy cores with just a little gas on top, while others have tiny cores with bloated hydrogen/helium atmospheres. Since we think gaseous planets all form more-or-less the same way, it’s hard to explain this wide range of internal structures.

Inamdar and Schlichting explore the possibility that giant impacts between young planets in these systems could account for this diversity. By applying a simple 1-D hydrodynamic model, they show that these massively violent collisions could easily remove large amounts of atmosphere from the young planets.

Whether a certain planet experienced such a collision depends in a stochastic way on the initial conditions and gravitational interactions in these chaotic young planetary systems. So some planets would have experienced large collisions that removed a lot of their atmospheres, giving a high mean density, while others didn’t, leaving them low-density.

These same kind of planetary collisions shaped the diversity of planets in our own solar system. For example, the Earth’s Moon formed as the result of a collision between the proto-Earth and Mars-sized object, named Theia. Uranus probably got its unusual tilt from a collision with an Earth-sized object early in its history.

So even though most extrasolar planetary systems we know about don’t resemble our own, the results from this study show the same processes shaped them, and planets everywhere probably experienced a violent adolescence.

Although the source of energy powering the volcanoes geysers** was not (and is still not) understood, their implication was clear: Enceladus could have a sub-surface ocean, like his big sister Europa.

The follow-up discovery of salty particles in the geysers all but clinched the existence of a sub-surface water source. Salt in the eruptions would probably require liquid water to dissolve rocky materials to produce the salt. But it wasn’t clear whether the sub-surface source is a small pocket of water directly beneath the geysers or a more global-scale ocean.

In our journal club today, we discussed a paper that points to the existence of a global ocean. The recently published study from Peter Thomas and colleagues analyzed Enceladus’ rotation to study its internal structure. By tracking the motion of hundreds of control points on Enceladus’ surface, they found that its outer icy crust oscillates back and forth during its rotation much more than it should if the moon were solid all the way through.

Thomas and colleagues show that the large oscillations they found (called libration) require a large layer of fluid within Enceladus to lubricate the space between its outer, icy shell and its rocky interior. Otherwise, the moon would oscillate a lot less. Difficult to say exactly, but Thomas and colleagues estimate the ocean could be as thick as 30 km beneath an 20-km thick icy crust.

Just as Europa’s ocean, a sub-surface ocean in Enceladus could represent an enormous harbor for life. Even on this tiny moon, such a deep ocean is only about a tenth the size of the Earth’s oceans*.

Although Enceladus’ surface would be a tough neighborhood for life, sub-surface biota (if they ever evolved) would be protected by a thick layer of ice from the vacuum of space and interplanetary radiation. In fact, this same radiation could impinge on the surface and produce a steady supply of biologically useful oxidants, which could then trickle down into the subsurface ocean and help power the alien biosphere.

Over its 4.5 billion year history, the Earth has probably experienced at least one, maybe more, severe glaciation events, during which ice covered more or less the whole Earth’s surface.

These events were probably very challenging for ancient Earth life, and some of the best documented events occurred at the end of the Neoproterozoic, about 540 million years ago, probably driving mass extinctions and possibly kicking off the Cambrian explosion in animal life.

In her presentation today, Prof. Dehler discussed her extensive field and laboratory studies of statigraphic deposits in the western US, laid down about this time in Earth’s history, deposits she calls ChUMPs.

While the ChUMP layers were building up, the Earth was going through dramatic geological and climatic variations. The ancient supercontinent of Rodinia was breaking up, and Dehler can see evidence of volcanism that accompanied this rifting as huge excursions in carbon isotopes locked up in the ChUMP deposits.

The carbon dioxide injected by these large eruptions probably also drove climate change, raising global temperatures, which caused rapid weathering of rocks that locked up the excess carbon dioxide in the rocks.

With so much CO2 sucked out the atmosphere, it’s possible the Earth then underwent rapid cooling, and this is what could have driven Earth into a snowball state during the Neoproterozoic.

However, the story told by the rocks and their isotopic compositions is complicated, and it’s not entirely clear whether the enhanced volcanism preceded the snowball Earth event: The ages attributed to the rocks still have uncertainties that are too large to be sure.

In any case, Dehler presented a compelling, complex story about the Earth’s ancient history, meticulously pieced together, one mineral grain at a time.

These lanes of darkened martian surface occur seasonally, usually on the slopes of craters facing the Sun, and have been observed on Mars for a few years. Their seasonal occurrence on Sun-facing slopes pointed to the possibility that they result from flowing water, but since flowing water on Mars could be such a huge discovery, scientists have been very cautious about their origins; hence the obscure name.

In the recent paper, Ojha and colleagues analyzed high-spatial resolution observations of Mars from the HiRISE camera to carefully localize the flows on Mars’ surface. They then analyzed infrared observations from HiRISE’s sister instrument CRISM to look for spectral evidence of salts dissolved in water.

The salts they looked for aren’t like table salt but instead consist primarily of perchlorates. These chemicals can be toxic for us, but they are well-documented on Mars. Ohja and colleagues showed that the lineae exhibit spectral features consistent with dissolution of perchlorate salts.

Early in the solar system’s history, the life-giving soup of chemicals that rained down in meteorites on Earth probably fell on Mars. Conditions on early Mars could have been similar to Earths’, so life could very likely have arisen. Whether life still exists on Mars may just be a question of whether it could escape to subsurface oases bearing liquid water, whose existence is strongly suggested by this new discovery. It may be only a matter of a few years before we find life on Mars.

In attendance at journal club this week were Jennifer Briggs, Karan Davis, Hari Goppalakrishnan, Tyler Gordon, and Emily Jensen.